The invention relates to an improved lithium-magnesium oxide catalyst for converting methane to ethane and ethylene, and the method of making the catalyst. Specifically, the catalyst is made using a sol-gelling preparation technique. A solution of a magnesium alkoxide in an alcohol is prepared. The magnesium alkoxide solution is then mixed with a solution of a lithium compound in an alcohol. Preferably, chlorine is introduced to the mixture. The magnesium alkoxide in the mixture is hydrolyzed to form a gel, and the gel is then calcined to generate the solid catalyst. A catalyst prepared by this method achieves a greater conversion and superior selectivity at lower temperatures than conventional catalysts used for converting methane to ethane and ethylene.
Methane and ethane are readily available chemical feedstocks. They derive from various sources such as natural gas, anaerobic digestion of organic material, and as byproducts of many chemical processes. Methane and ethane are low molecular weight alkanes which exhibit high chemical stability. Because of their high chemical stability they are difficult to convert into higher molecular weight hydrocarbons. Moreover, it is difficult to convert ethane into ethylene.
In particular, it is desirable to convert methane and ethane to ethylene. Ethylene, unlike methane and ethane, is readily converted into higher molecular weight hydrocarbons. For example, ethylene is useful in synthesizing numerous other materials such as plastics. Consequently, there is a great desire to develop processes for converting readily available methane and ethane to more desirable hydrocarbons such as ethylene.
Substantial amounts of research have been conducted on catalytic processes for converting methane and ethane to higher molecular weight hydrocarbons. Keller et al. 73 JOURNAL OF CATALYSIS 9-19 (1982). For example, U.S. Pat. No. 4,826,796 teaches a catalyst for oxidative coupling of methane to produce ethane and ethylene. (Column 2, lines 5-12). Specifically, the patent describes a metal oxide catalyst which has the formula: A.sub.x B.sub.y C.sub.z O.sub.q where A is an alkali metal selected from lithium, sodium, potassium, rubidium, and cesium; B is a cation which has an ionization state one greater than the ionization state of C, and B is selected from the group consisting of boron, aluminum, yttrium, and lanthanum; C is selected from the group consisting of magnesium, calcium, barium, and zinc; and O is oxygen. Z is 1, X ranges from 0.00-0.25, Y ranges from 0.01 -0.25, and Q is the number necessary to maintain charge balance for the oxygen. (claim 1 at column 6, lines 26-35). Metal oxide catalysts of these formulas provided conversions of up to 21.7 percent. (Table 1 at column 5, lines 10-25).
Conventional technology for converting methane has achieved ethane plus ethylene yields of only 20 weight percent. This yield is too low for economical commercial processes. In particular, a need exists for improved catalysts to convert methane and ethane selectively to ethylene at yields that are commercially feasible.